CPU and GPU - does CPU work with or alongside the GPU?
If you've selected to use both the GPU and CPU for rendering, the GPU will work on a single tile while each CPU thread will also work on a tile. In this case, you will see the GPU rendering single tiles much quicker than the CPU threads can render their respective tiles. However, if your GPU does not have enough video memory to render a scene, Blender will use your CPU to render. Unless you're working with massive scenes, I don't think that's the case here.

To find specific render times, go to Blender OpenData to view the benchmarked times for various devices. Note that this is for that specific device(s) being used to render. Since the hardware you're interested in isn't in the top 50, go to the Raw Data section and find the relevant benchmark times in the JSON file (which you can do by importing the file to Excel). It's not the most user-friendly, but you can also search for benchmarks 3rd parties have done and posted online.

I do not believe that your render times will be halved. To my understanding, if you're only using your GPU to render, the CPU will be doing physics calculations (hair, collisions, object transformations etc.) while the GPU deals with rendering actual scene once the CPU has calculated the aforementioned things.

When it comes to multicore processing, things get a bit tricky. Many calculations cannot be done in multiple threads and must be sequentially calculated. For example, consider an object parented to another during an animation. Blender must first calculate the parent's position, and only then can it calculate the position of the child relative to the parent's position.